For my summer project i'm building a self balancing robot that balances on a ball. I have two 24v brushed dc motors powering the motor with compactRIO hardware running to a sabertooth 2X25 amp driver.

Currently i am using a huge 12v (ideally needs to be 24v) car battery off the robot to power it. However i want to have the robot self contained so need a lighter more portable battery. The motors nominal current is around 4 amps and they peak at 16amps.

Does anyone know of a good battery that is safe (if the robot falls the battery cannot break etc), either 12v or 24v and can supply the motors with enough current? My budget is funded by the university so the cheaper the better but if needs be i could go to £50 or maybe even £100.